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http://people.clarkson.edu/~wwilcox/Design/refhysys.htm
UniSimDesign Tutorial for CHEE470
Queens University Department of Chemical Engineering
2006
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Introduction
UniSim (formerly HYSYS) is a program that can be used todesign chemical plants. It is built around:
a library of the physical properties of a large number of
chemical species
a set of subroutines to estimate the behavior of many types
of plant equipment (heat exchangers, reactors, etc.)
a graphical user interface to accept specifications for the
case, and display results
The user describes the process in terms of pieces of equipment
interconnected by process streams, and the program solves all
the mass/energy/equilibrium equations, taking into consideration
the specified design parameters for the units.
It is a very complex system, and there is no way that this tutorial
is going to demonstrate all of the features. The features that willbe shown are the ones that will prepare you to tackle the plant
design assignment in CHEE470.
Like most programs of this type, operations can be done in
different ways. In general, this tutorial will only describe one
way. You will find other methods in the UniSim documentation,
but the ones shown here are best suited for people who are newto the program.
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Hints for Success in Modeling
1.
Build the model one step at a time. People new to this
instinctively want to start by adding many streams and unitoperations, and then try to get the whole mess working.
This is futile. Add the elements one at a time. Get one
working before you go on to the next one.
2.Save a whole series of backups, not just the latest working
version. Disk space is cheap. If you get into trouble, you
may find that it is difficult to restore the model to its
previous state. Often you are better to retrieve a previous
version and update it.3.
Put meaningful names on all streams and units. Debugging
is difficult when you are trying to remember if stream S22
is the distillate or the bottoms in a distillation column.
4.
If a piece of equipment does not work although the
parameters all look reasonable, try deleting the unit and
reconstructing it.
Steps in Developing a Model of a Chemical Process
1.Select the units that you want to work with. Do you want
kilograms and C, or pounds and F?
2.Select the thermodynamic methods that will be used for
predicting physical properties. The decision should be
based on the type of chemical species involved.
3.
Specify the chemical species that will be present in the
process. At this stage you may be given some advice about
the ability of the selected thermodynamic method to handle
these chemicals.
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4.
If the process involves reactions, provide information such
as stoichiometry and kinetic constants.
5.
Build the model by adding streams and equipment one at a
time.6.If the process contains recycle loops, deal with closing
them.
7.Use the UniSim utilities to get additional information such
as the mechanical design of distillation column trays.
8.Print a report describing the results of the simulation.
The Methanol Process
Methanol can be made from hydrogen plus carbon monoxide
and/or carbon dioxide.
2 H2+ CO => CH3OH
3 H2+ CO2=> CH3OH + H2O
Recent studies suggest that the first reaction actually proceeds asCO + H2O => H2+ CO2 (the water gas shift reaction)
followed by the second reaction.
For this exercise we will work with the simplest versionthe
second reaction only. By the way, running this reaction
backwards provides a method of operating a hydrogen fuel cell
with methanol as a feed.
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The following diagram shows the process we will work on. It is
important to recognize that this is not suggested as a good way
to make methanol. The design has been formulated to
demonstrate many key aspects of UniSim, without gettingoverwhelmed by detail.
A mixture of H2and CO2is heated to the required temperature
and fed to a stirred reactor. As noted above, the reaction is 3 H2
+ CO2=> CH2OH + H2O.
The product of the reaction is partially condensed. The vapour
(mostly H2and CO2) is compressed and recycled back to the
beginning of the process. The liquid (mostly CH2OH and H2O)
is fed to a distillation column.
The column produces a product stream (mostly methanol) and a
waste stream (mostly water). The product is cooled to a
temperature that is reasonable for storage. A pump is required
provide cooling water for this heat exchanger.
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Setting up Data for the Model
Open UniSim.
Click on the new file icon and get:
Units
We will use the default SI units, so no action is required. In
future cases you may want to use different units. See section3.2.2 of UniSim Design Tutorials and Applications.pdf.
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Thermodynamics
Click on the Fluid Pkgs tab.
Fluid Package is UniSims terminology for a collection of
data that includes all the thermodynamic, component, and
reaction parameters required to run the model.
It is possible to have more than one package in a model. For
example, it would be possible to use one thermodynamics model
in the reactor, and another in the distillation column. We will
just have one package.
Click on Add and scroll to find SRK. This selects the
Soave-Redlich-Kwong method, a popular equation of state
model.
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Note that the default name Basis-1 is given to the package,
and our components will go into Component List-1.
Components
Close the fluid package window and click on the Components
tab of the Simulation Basis Manager.
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Now, select Component List 1, click on View and enter
CO2 into field Match.
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Now click on Add Pure, and do the same operation with
Hydrogen.
Lets do methanol differently. Select the Formula optioninstead of Full Name / Synonym and enter CH4O>
Do the same with H2O.
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We now have all the components we need.
This would be a good time to save the case. Close the
components window and click on the Enter Simulation
Environment button of the Simulation Basis Managerwindow. Do the usual File=>Save operation and call the file
tutor01.
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Reactions
Now we need to return to the Simulation Basis Environment.
Click on the beaker.
In the Simulation Basis Manager window, select the
Reactions tab.
Click here
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Now, click on Add Rxn. When the Reactions window
appears, select Kinetic and click on the Add Reaction
Button.
A Kinetic reaction is one for which we will supply the kinetic
constants that define the rate of reaction. This allows us to size
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the reactor. If we were only interested in simuating the heat and
mass balances, we could use the simpler form, a Conversion
reaction. Then we would only have to define the percent
conversion.
Click on **Add Comp and select the components as shown.
Now fill in the following data:
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The significance of the numbers should be obvious from the
definition of the reaction:
3 H2+ CO2=> CH2OH + H2O. Note that Rxn-1 has been
replaced by a more meaningful name Methanol Reaction.
Now look at the Basis tab.
Basis = Molar Concn means that the reaction rate equation usesconcentrations of the reactants in moles/m3.
Base Component = CO2 means that the reaction rate equation
describes the rate of consumption of CO2, not consumption of
H2or production of Methanol (since CO2consumption =
methanol production, methanol could be specified here).
Rxn Phase = LiquidPhase means that the reaction takes place in
the liquid. Since we will not have any liquid in the reactor, this
is not helpful. Change it to VapourPhase.
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Finally, change Rate Units from kgmole/m3-s to
kgmole.m3-h. Since everything else in the modle is in units of
hours, it is best to be consistent.
The important consideration is to ensure that the treatment here
is compatible with what was used in generating the constants
describing the reaction rate (probably from lab data).
Now go to the Parameters tab and enter the following values
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These are the kinetic constants for an Arrhenius equation: k = A
e(-E/RT)
Note that the bar in the lower right has turned green and saysReady. This means that all the necessary data have been
supplied, and are valid (that does not necessarily mean correct).
Close the window and return to the Simulation Basis Manager.
Note that Methanol Reaction has been added to the list of
reactions.
UniSim has put together data for a set of reactions for us called
Global Rxn Set. It only contains one reaction, Methanol
Reaction, but we could add others (e.g. side reactions that
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produced small quantities of ethanol and acetone). If our model
contained an unrelated group of reactions (e.g. if we put a water
gas shift reactor on the front of the process) we could create
another Reaction Set. In this way we could model differentreactors using different reactions.
To see what is in our Reaction Set, click on View Set.
This does not tell us anything that we do not already know, but it
confirms that our reaction is really there.
The next step is very important, and it is easy to forget to do it.If you find yourself unable to model a reactor because the
reactions you need do not exist, it is probably because you
forgot this step.
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At this point UniSim has put Methanol Reaction in Global
Rxn Set, but it has not put Global Rxn Set in Basis-1.
When building the reactor it will look in Basis-1 for the
reaction data.
Back in Simulation Basis Manager click on Add to FP.
Now click on Add Set to Fluid Package.
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Note that Basis-1 is now listed in Assoc. Fluid Pkgs.
We have now finished the job of supplying the data for physical
properties, components, and reactions. Note that we have left thenames Global Rxn Set, Component List-1and Basis-1
with their default values. If we had more than one basis,
component list, or reaction set, we would have been wise to
change the names to something more meaningful, just like we
changed Rxn-1 to Methanol Reaction. But if there is only
one of each, we are not going to get confused about which one
we are dealing with in the model, and this is unnecessary.
To proceed with building the process model, click on Return to
Simulation Environment.
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Building the Process Flow Diagram
Always save the case at this point in the development. In thisway, regardless of how screwed up the model gets, you can
always go back to a valid case
Do a Save followed by Save As tutor03.
We now have a blank screen on which we can start to build a
PFD (Process Flow Diagram) that will define the process.
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Feed Section
While we are working on the PFD we will require
the use of the unit operations palette as shown onthe left. If it is not present, do
Flowsheet=>Palette.
The first thing we want to do is create the feed to
the system. Double click on the blue materialstream icon to bring up an empty stream window.
Note that this is reminding us that the properties are being
estimated with the Basis-1 package.
Material Stream
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Enter the following data:
Stream Name = Feed
Temperature = 40 C Pressure = 4000 kPa
Mass Flow = 1000 kg/h
To finish the stream definition, we need to specify the
composition. Click on Composition at the left side of the
window.
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We want to specify a stoichiometric ratio, so mole fraction is the
correct units.
Specify:
CO2 = .25 H2 = .75
Normalize
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Go back to the Conditions window.
Note that the stream is now fully defined, and UniSim has
calculated the variables that you did not specify. From now on
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we will only be able to change the blue values, not the black
ones.
You may have noticed that there were two stream icons on the
palette. The first (blue) is a material stream
going from one piece of equipment to another. The second (red)
is an energy stream. These will be handled by UniSim. The
energy used or generated by equipment will be displayed in
these streams. This information will be useful in costing the
utilities (steam, electricity etc.) used by the process.
Now let us add a mixer to combine the feed with the recycle
stream. In most cases, but not all, a mixer is not a piece of
equipmentjust two pipes coming together. Double click on the
mixer icon in the palette.
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For Name, specify Recycle Mixer. Then click on
in the Inlets list. You will be presented with a
list of acceptable streams.
Select the only item in the list: Feed.
Now we want a second input stream, so enter Recycle in the
line below Feed.
Next, click on in the Outlet box, and you will see that there
are no candidate streams. So enter the name Mixed.
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Why is it Not Solved? Because we have not described the
other input stream Recycle. To do this, go to the Worksheet
tab because it will allow access to all the streams connected to
the mixer.
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At this point we need an estimate of the recycle stream. How
this is arrived at will be different for every process, but usually
requires knowing what the conversion per pass in the reactor
will be. For this exercise, use the following values. Temperature & pressure = same as stream Feed
Molar Flow = 200 kgmole/h
Next, go to the Composition window and enter the following
values.
Now UniSim has sufficient data to do the necessary calculations.
Let us look at the main screen.
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Since all of the streams are dark blue, UniSim is happy.
The next step is to heat the mixed stream up to reactor
temperature. In the palette, double click on the heater icon.
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Note that the Basis-1 properties package will be used here. If
another one was available we could select it.
Enter the following data:
Name = Feed Heater Inlet = Mixed (from dropdown list)
Energy = Heater duty
Outlet = To Reactor (a new stream)
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Next, go to the Parameters window and enter a value for the
pressure drop across the heater. 50 kPa is a reasonable number.
An alternative would be to leave this empty and specify the
pressure of the output stream in the next step. But it makes more
sense to specify the pressure drop rather than the outputpressure. Consider what would happen if the upstream pressure
changed.
Do not enter a value for duty. In a moment we are going to
specify the output temperature, and it is not possible to specify
both. Of course, there are situations in which we might want to
define duty rather than temperature, but this is not one of them.
If you have used other process simulators such as PRO/II you
will remember that the output temperature specification is
treated as a parameter of the heat exchanger rather than the
output stream. You will need to reorient your thinking.
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To specify the temperature we need to go to the Worksheet
tab. Enter a value of 200C.
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Now everything can be calculated.
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Before proceeding to tackle the reactor, it would be good idea to
save the case again. Do Save followed by Save As tutor04.
Reaction Section
We will model the reactor as a stirred vessel with sufficient
cooling to maintain the output temperature at the same value as
the input. On the palette, double click on the CSTR icon.
Fill in the data, using the same techniques that were used inprevious units. Although we will not have any liquid leaving the
reactor, UnSim requires a liquid stream just in case some is
generated. The flow rate will be set to zero.
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Note the message about the need for reaction data. Go to the
Reactions tab and select the reaction set from the dropdown
list (only one option exists).
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Now we need to specify the size of the reactor. At this stage we
do not know how big the reactor should be. The best thing to do
is set the dimensions unreasonably large. This will drive thereaction close to the equilibrium point, and provide a stable
environment when we come to deal with the recycle. Go to the
Rating tab, and enter numbers like the following.
We also have the ability to specify a pressure drop across the
reactor. That is done in the Parameters section of the Design
tab.
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Now it is complaining that it does not have enough data to
calculate duty (the cooling rate). We have two choices:
Specify a cooling rate in kJ/h (the Duty field in the
window shown above)
Specify the outlet temperature (we will do this in theWorksheet tab)
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Now it is happy. If you want to see what is happening in the
reactor, look at the Composition section. The output has more
methanol and less CO2& H2than the feed.
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Recycle System
The next step is to condense the methanol and water, and return
the gases as recycle.
First, we need a cooler to condense the liquid by cooling it to
40C. We will do this in the same way that we defined the feed
heater, but will select a cooler instead of a heater fromthe palette. Assume a pressure drop of 50 kPa.
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Note that the output, Condensed Mixture, is two phase. We
need a separator to isolate the two phases. Select a Separator
from the palette and attach the streams as shown.
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Note that this is all that UniSim wants from us. But we have
made an unconscious decision to accept a default. Look in the
parameters section.
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This is telling us that both outputs are at the same pressure as the
input. There are other parameters we could specify, but they are
not necessary for our case.
Before we recycle the vapour, we need to split off a small purge
stream to prevent buildup of noncondensible gases in the loop.
In the palette, the device we need is a Tee.
Give it the following parameters.
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This may look like it is putting nothing into the purge stream.
The problem is too few digits in the display. Look in the
Worksheet tab.
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This would be a good point to see what the process looks like. It
should look something like this.
If the equipment has got pushed around on the screen do a
PFD=>Auto Position All operation and the units will be
arranged in a logical order.
The stream Recycled is at a lower pressure than the feed, so
we need a compressor to get it back to the mixer.
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In the Parameters section, accept the default efficiency of
75%.
Then, set the output pressure in the Worksheet tab.
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We are about to close the recycle loop. In doing this, it is not
uncommon to end up with an unworkable model. Always save
the case before closing the loop. Do a File Save and File
Save As tutor05.
Set up a Recycle unit from the palette and specify the
two connections.
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The PFD should now look something like this:
Before we finish with the reaction section there is one more job
to do. Earlier we had set aside the question of how big the
reactor should be and just set it very big. For all practicalpurposes the stream leaving the reactor is at the equilibrium
concentration. There is a tradeoff here between reactor size and
the size of the rest of the equipment in the recycle loop. If we
reduce the size of the reactor, the conversion per pass will fall.
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This will require a higher recycle rate, leading to larger heat
exchangers compressor separator etc. Somewhere there will be
an minimum equipment cost (operating costs will also be an
issue).
For this exercise we will say that previous designs have shown
the optimum is about 90% of the equilibrium conversion, and
the height of the reactor should be twice the diameter.
Look at the Results section of the Reactions tab.
We should aim for a conversion of 46.03 * .9 = 41.4
Vary the reactor size and see what happens to the conversion.
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Diameter Height Conversion
1 2 42.65
.9 1.8 41.54
.89 1.78 41.40
We will end up with something like this.
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Do a File Save and File Save As tutor06.
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Product Separation Section
All of the equipment simulations we have done so far have been
ones in which any reasonable set of parameters would lead to a
calculation that works. For example, as long as we did not do
anything ridiculous like specify a temperature of -500C, we
would get a valid heat exchanger design. The answer might not
be what we wanted, but we would get an answer.
With a distillation column we might not even get an answer.You may recall from earlier courses in distillation that some
separations are only possible with a number of equilibrium
stages above a certain value, or a reflux ratio above a certain
value. Some configurations just do not work.
In most cases we want to design a column to meet certain
concentration specifications. In our case they are: 97% of the methanol entering the column leaves in the
product
The methanol product contains 1% (by mass) water
There are two specifications because a simple column with two
products and feed, pressure, number of stages, location of feed
tray specified has two degrees of freedom.
In some cases we could go directly to a model with these specs.
In general, it is safer to start by creating a case that works (even
though it is not what we want) and then migrate to the case we
want. The configuration most likely to work is specifying:
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The flow from either the top or the bottom of the column
The reflux ratio
We will do it this way.
With UniSim you must have certain information lined up before
you start putting the column together. You will be taken through
a series of windows that you must complete. You cannot break
out and come back later. We will need the following
information:
Name of feed stream (Liquid)
Number of stages (this separation is easy so try 10)
Location of feed (we have nothing to base this on so put it
in the middlestage 5)
Pressure in the condenser (use 1000 kPa)
Pressure in the reboiler (use 1015 kPa)
Will we take the product off as a liquid or vapour (vapour,
do you think it would be a good idea to attempt to condense
hydrogen?)
A starting value for reflux ratio (3 is suggested)
A starting value for the distillate rate (19.729 kgmoles/hr -
see the chart below)
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Double clicking on the Distillation Column icon on
the palette will bring up this:
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Enter the following data:
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We want to take all of the top product as a vapour. A partial
condenser allows both liquid and vapour. We have specified two
streams, but will set the flow of Dummy to zero at a later step.
Note that the addition of this third output stream increases the
degrees of freedom by 1. At the same time a constraint (flow =
0) is added and the net effect is that we still need to provide two
specifications. Another way of looking at it is that a stream with
zero flow does not really exist.
Next, specify the pressure profile.
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In case with complex vapour-liquid equilibrium relationships,
estimating the temperature profile can help the program to
converge on the right answer. Our case does not require this.
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Note that we have set Dummy to zero. We now have all the
parameters specified. Click on Done.
Click on run and get a valid case (but not the right case).
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To complete the job, go to the Specs section.
If you examine the Column Specifications you will see that
the first three are Active (the calculations force them to be
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met) and the last two are alternatives that are Inactive
(ignored). We will leave Distillate Rate alone (the dummy
liquid stream) while we replace Reflux Ratio and Ovhd Vap
Rate by our two composition specs.
First, create the two new specs but leave them inactive. Under
Column Specifications click on the Add button and start on
the methanol recovery spec.
Select Component Recovery and click on Add Spec(s).
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Recovery means the fraction of the component in the feed that
goes to the specified stream.
Now do the methanol concentration in water spec.
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Now change the spec value to whatever it happens to be at the
present time, and make the spec active.
Note that Degrees of Freedom has changed from zero (the
correct number of specs are active) to -1 (too many are active).
Deactivate Ovhd Vap Rate and look at Comp Recovery.
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Now do the same with the other spec.
We now have two too many specs. Deactivate Reflux Ratio
and Ovhd Vap Rate and the case will run.
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We have switched to a new set of specs, but the case has not
really changed. All of the concentrations, temperatures etc. are
the same as before. This makes the transition more or less
foolproof.
The final step is to change the values of the specs to what we
really want.
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The PFD should now look like this:
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Now do a File Save and File Save As tutor07.
Before leaving the column simulation, let us review the steps we
went through to get the case we wanted.
1.
Estimate the distillate rate from the feed composition and a
knowledge of which components are to go out the top.
2.
Pick a starting value for reflux ratio. Other programs use a
default of 3, and that works most of the time.
3.
Build a model with distillate rate and reflux ratio specs.4.Get this model to converge. It may be necessary to change
parameters such as number of stages, feed location, reflux
ratio etc.
5.Build the specs that you really want and set their values to
those in the working case.
6.Activate these new specs and deactivate the distillate/reflux
ones. The model should converge.7.
Change the values of the new specs to match what you
want in the column. If there is a large change, you may
want to do it in stages.
.
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Product Finishing
The final step in the process is to condense the methanol productand prepare it for storage. Before we start on the condenser, we
need a source of cooling water. It will be taken from a storage
tank and pumped to 600 kPa. Initially, we will set the flow rate
very high (10,000 kg/hr) to ensure that we have enough for the
heat exchanger. During the heat exchanger design we will
reduce this to a reasonable value.
Set up stream Water Source in the same way that we createdFeed.
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Now add a pump.
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As with the compressor, we set the output pressure in the output
stream. Use a value of 600 kPa.
Now we can start on the heat exchanger.
UniSim provides three levels of detail for heat exchangerdesign:
1.
The simple method, using a Heater or Cooler just does
the heat balance necessary to take a stream to a specified
temperature.
2.The intermediate method deals with transfer of heat
between two streams. A value of UA (overall heat transfer
coefficient * heat transfer area) is specified and UniSimcalculates the two output temperatures. If an estimate of U
is available, the area can be calculated and used for a crude
cost estimate. The recommended procedure is Exchanger
Design (Weighted).
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3.
The most rigorous method is Steady State Rating. With
this procedure the mechanical design is specified (tube
number/dimensions/spacing, shell number and
configuration, etc.), UniSim estimates U and calculates thetwo output temperatures. We will defer the use of this
method until we have learned more about heat exchanger
design.
Let us design the condenser by method 2, using the Heat
Exchanger unit. There are two requirements for thedesign. The process stream should exit at 40C, and the cooling
water at 45C.
Start by specifying the following data. Note that the process
stream is going through the tubes and the cooling water through
the shell.
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In the Parameters section select the Exchanger Design
(Weighted) method and supply pressure drop estimates. Then
adjust the UA value so that the stream Final Product has a
temperature near the target of 40C, as seen in the Workshop
tab.
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We are not finished because the cooling water exit temperature
is too low because of the high flow rate. UniSim has a unit
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called an adjust. It is something like a process controller
in a plant, but it manipulates the model, not the process. It tells
us nothing about the dynamics of the process. Like a controller,it changes the value of one parameter in order to bring another
parameter to a specified value.
First, under Adjusted Variable click on Select Var and
specify that we want it to manipulate the cooling water flow.
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Then, under Target Variable click on Select Var and specify
that we want to control the outlet water temperature.
The value we want it to settle out at is 45C.
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Ignore the warning about Unknown Maximum. If there is a
problem in converging, changing the values in the Parameters
tab may help.
This message is common the first time the model is run. Click
on Yes.
Now look at the condenser worksheet tab and verify that both
outlet temperatures are correct.
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To finish the design there is one more piece of equipment to add
a tank to store the product.
Connect the input stream Final Product and supply names for
liquid and vapour outputs.
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Also, you can specify the pressure in the tank.
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Note that there is some vapour (mostly CO2and hydrogen) that
will be vented from the tank. Final Product was a two phase
mixture.
The final model should look like this.
Now do a File Save and File Save As tutor08.
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Modeling Tools
Having completed the process model we will now take a look atsome of the facilities in UniSim that allow us to generate
reports, do additional design tasks, and help with model
development.
Reports
It is possible to generate, view, and print reports on the whole
model or specific pieces of equipment. Let us get an overallview of the streams in the model. The Tools menu has a
Reports item.
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Click on Create to generate a new report.
This is the result of Create, Insert Datasheet. A report on the
whole model is selected.
To view the report click on Preview. Here are some samples
of what is in the report.
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If you click on Print you can print a copy.
Later, after modifying the model, you can come back through
the Tools=>Report route and print an updated copy. You do
not have to redefine the report content at that time.
Column Tray Design
UniSim has a utility to do a mechanical design of distillationcolumns, both trayed and packed. Go to the Tools menu and
select Utilities.
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We want the Tray Sizing utility.
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To specify which section of which column to do, click on
Select TS, and select the column to be sized.
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Now click on Auto Section and let UniSim decide what needs
to be done.
Here we have selected valve trays. In this utility there are a lot
of parameters that can be specified, but we will just accept thedefaults.
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Click on Complete AutoSection.
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Note that UniSim has decided to do the design in two parts.
Section_1 is the trays above the feed tray, Section_2 isbelow. Because the flow rates are different in the two sections,
one part can be made smaller than the other, if desired.
Look at the Results section of the Performance tab.
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This shows the recommended design. Note that the lower
section could be made with a smaller diameter than the top.Building a top-heavy column is not a good idea, although the
reverse is sometimes done. The best thing to do here is to build
the whole column at the larger diameter. (.6096 m = 2 feet).
Note that the dimensions, although displayed in metres, are
actually selected from standard sizes in feet.
Useful Techniques in Developing a Model
Here are a couple of techniques that can make it easier to
develop models.
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Suppose we are told that, over the lifetime of the catalyst in the
reactor, its activity can fall as much as 20%. And we are asked
the question Will this affect the concentration of the feed to thedistillation column. It would be helpful if we could work on the
reactor and the column feed stream at the same time so that we
would not have to flip back and forth between them.
Double click on the reactor icon and move to the Sizing
section of the Rating tab. Double click on the stream Liquid
and move to the Composition section. Select Mole Fraction
as the basis.If you minimize the PFD window, you should get something
like this.
Looking at the dimensions of the reactor, something seems
wrong. The height/diameter ratio is when it should be 2.
Although it does not affect the reaction calculations (only the
volume is significant), we should fix this before proceeding.
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We want to change two numbers, but we do not want UniSim to
recalculate the model until both numbers are changed. The
intermediate case with one number changed would send the
model off into unexplored territory and might not converge.
At the top of the main window you will find this: The
left (green) light indicates that calculation is activated. Click on
the right (red) light to deactivate calculation.
Now change the numbers. A little arithmetic shows that we want
D= 1.121, H = 2.242 to get the same volume. Change both ofthese and then click on the green light.
HINT: In working with UniSim and the program seems to seize
up, check to make sure that the red light is not on.
At this point we should have something like this:
Now, reduce the height 20% (to 1.794) to simulate the effect of
catalyst decay.
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Here we have our answer. There is no significant change incomposition.
Change the height back to its original value, close the two
windows, and restore the PFD window.
We are now finished with the development of the model. Save
the file (it should be tutor08) and exit UniSim.
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Appendix
UniSim Customization
In Tools=>Preferences=>Resources=>Colours
Set PFD Background to white
Set PFD Label Text to black
Set PFD Annotation to black
In Tools=>Preferences=>Simulation
Uncheck Confirm Before Adding if Active Correlations
are Present
Uncheck Enable Cross Hairs on PFD
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